Water Hammer Report

Faculty of engineering Mechanical Engineering Department

Water Hammer Report Dr/Hassan warda

The report was written by :

Ahmed Mohi Eldin Fahmy Ahmed Mustafa Mohamed Youssef Ahmed Ragab Fathy Ibrahim Ahmed Mohamed Abdel Rahman Mohamed Ahmed Mahmoud Abass Ashry Ahmed Mohamed Ahmed Waley

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Contents

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• Introduction • Definition of water hammer • Casues

• Features • Damage caused by water hammer • Equation of water hammer pressuare • Instantanous valve closure • Methods to reduce or eliminate water hammer • Visualization • CONCLUSION • References

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Introduction Water hammer refers to fluctuations caused by a sudden increase or decrease in flow velocity. These pressure fluctuations can be severe enough to rupture a water main. Potential water hammer problems should be considered when pipeline design is evaluated, and a thorough surge analysis should be undertaken, in many instances, to avoid costly malfunctions in a distribution system. Every major system design change or operation change such as the demand for higher flow rates should include consideration of potential water hammer problems. This phenomenon and its significance to both the design and operation of water systems is not widely understood, as evidenced by the number and frequency of failures caused by water hammer .

Definition of water hammer Water Hammer is a term used to define the destructive forces, pounding noises, and vibrations which develop in a piping system when a column of non-compressible liquid flowing through a pipe line at a given pressure and velocity is stopped abruptly. The tremendous forces generated at the point of impact or stoppage can be compared in effect to that of an explosion .

Casues A water transport system’s operating conditions are almost never at a steady state. Pressures and flows change continually as pumps start and stop, demand fluctuates, and tank levels change. In addition to these normal events, unforeseen events, such as power outages and equipment malfunctions, can sharply change the operating conditions of a system. Any change in liquid flow rate, regardless of the rate or magnitude of change, requires that the liquid be accelerated or decelerated from its initial flow velocity. Rapid changes in flow rate require large forces that are seen as large pressures, which cause water hammer.

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Water flowing through a pipe has a definite amount of energy of flow This is known as kinetic energy and can be calculated by using the formula :

K.E : kinetic energy M

: mass of water which is flowing

V

: velocitry of flow

g

: acceleration due to gravity

When the flow of water in a system is abruptly stopped, this kinetic energy must be absorbed. In an unprotected piping system this energy is dissipated by straining and expanding the piping and various components in the system and is accompanied by a dangerous pressure rise in the system . Entrained air or temperature changes of the water also can cause excess pressure in the water lines. Air trapped in the line will compress and will exert extra pressure on the water. Temperature changes will actually cause the water to expand or contract, also affecting pressure. The maximum pressures experienced in a piping system are frequently the result of vapor column separation, which is caused by the formation of void packets of vapor when pressure drops so low that the liquid boils or vaporizes. Damaging pressures can occur when these cavities collapse. In conclusion most of the causes might be : 1. pump startup or shutdown; Pump startup can induce the rapid collapse of a void space that exists downstream from a starting pump. This generates high pressures , Pump power failure can create a rapid change in flow, which causes a pressure upsurge on the suction side and a pressure down surge on the discharge side. The down surge is usually the major problem. The pressure on the discharge side reaches vapor pressure, resulting in vapor column separation , and the formed vaccum may result in the pipe deformation. 2. valve opening or closing variation in cross-sectional flow. Valve opening and closing is fundamental to safe pipeline operation. Closing a valve at the downstream end of a pipeline creates a pressure wave that moves toward the reservoir. Closing a valve in less time than it takes for the pressure surge to travel to the end of the pipeline and back is called sudden valve closure. Sudden valve closure will change velocity quickly and can result in a pressure surge. 3. changes in boundary pressures , such as losing overhead storage tank, adjustments in the water level at reservoirs, pressure changes in tanks . 4. rapid changes in demand conditions ,such as hydrant flushing . 5. changes in transmission conditions ,such as main break or line freezing.

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6. pipe filling or draining ,as when air release from pipes 7. check valve or regulator valve action. 8-Improper operation or incorporation of surge protection devices can do more harm than good. An example is over sizing the surge relief valve or improperly selecting the vacuum breaker-air relief valve.

Features 1-The undertaking of a water hammer analysis, and selection of protection measures, should be an integral part during the design phase. There are now propriety water hammer programs available, which can assist designers in identifying potential water hammer problems and help in the selection protection measures. The use of these programs should be limited to experienced designers with intimate knowledge of water distribution systems. 2-The magnitude of transient pressures (or water hammer) and the time duration of the transient condition depends on the flow rate velocity, pipeline material and the system boundary conditions such as tanks, pumps, air valves, control valves and changes in pipeline diameter. 4- Steel pipe has pressure wave speed of 1000 m/s compared to 250 m/s for polyethylene pipe. The sudden closing of a valve with a pipe flow velocity of 1.0 m/s would generate a pressure change of 100 m head in the steel pipe compared to 25 m head in the polyethylene.

Factors affecting the pressure surge(Water Hammer) There are many factors that influence transient pressure surges, they include:            

Pipeline profile (particularly high points) Pipeline anchorage Type of pipe material (and presence of linings) Location of storage’s Type of check valves (some are vulnerable to valve slamming) Pump performance curves and operating speeds Rotational moment of inertia’s for pump/motor assemblies Pump station configurations Location and type of air valves Protection devices installed Configuration of piping network Valve types, sizes and their opening/closing speed 6

Damage caused by water hammer Damage to pipes, fittings, and valves, causing leaks and shortening the life of the system. Neither the pipe nor the water will compress to absorb the shock.

The main damages are :  Pipeline bursts and leaks • Pipelines can fail by buckling resulting from excessive vacuum during transient conditions in the case of thin walled large diameter steel pipe, low pressure rating plastic pipe and plastic pipes exposed to high temperatures. • Cement lining in steel pipes has spalled off the pipeline in situations where the pipeline is subjected to vacuum conditions accompanied by large pressure fluctuations. The exposed metal surface corrodes resulting in accelerated pipeline failure. • Asbestos Cement rubber ring joints have failed from vacuum pressures resulting from pump stoppages. The vacuum pressures have allowed air to enter the pipeline via the rubber ring joints and the joints have failed with time exposure.  Damaged Equipment • This may occur due to the violent movement of mechanical parts. Examples of these are check valves slamming shut following pump stoppages at multiple pump stations and the sudden closure of large orifice air valves when filling pipelines.

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Equation of water hammer pressuare Let "dv" is change in velocity in time "dt" as the valve is closed abruptly. In time dt an element of liquid, of length "Cdt" is brought to rest. The mass of the liquid compressed against the valve and comes to rest in time "dt" will be .

Multiplying both sides by ' a '

(

Ph : Water Hammer pressuare

ρ : density of the fluid C : speed of sound in the fluid V : Velocity of fluid

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Instantanous valve closure If the time for closing the valve "T" is assumed to zero, the valve closure is called Instantaneous valve closure. T is the time for closing the valve. Consider a pipe of length "L" leading from a reservoir and terminating in a valve at its downstream. When the valve is Instantaneous closed a pressure of magnitude "Ph" is formed and moves up with velocity "C". The wave undergoes reflections at the reservoir end as well as at the valve .

For t = 0, the pressure profile is steady, which is shown by the pressure head curve running horizontally because of the assumed lack of friction.Under steady-state conditions, the flow velocity is v0 .

The sudden closure of the gate valve at the down stream end of the pipeline causes a pulse of high pressure ∆h The pressure wave generated runs in the opposite direction to the steady-state direction of the flow at the speed of sound and is accompanied by a reduction of the flow velocity to v = 0 in the high pres-sure zone. The process takes place in a period of time0 < t < Tr , where Tr is the amount of time needed by the pressure wave to travel up and down the entire length of the pipeline.

At t = Tr the pressure wave has arrived at the reservoir. As the reservoir pressure p =constant, there is an unbalanced condition at this point.With a change of sign, the pressure wave is reflected in the opposite direction. The flow velocity changes sign and is now headed in the direction of the reservoir.

A relief wave with a head of - ∆h travels down stream towards the gate valve and reaches it at a time t = Tr . It is accompanied by a change of velocity to the value -v0 . 9

Upon arrival at the closed gate valve, the velocity changes from -v0 to v = 0.This causes a sudden negative change in pressure of -∆h.

The low pressure wave -∆h travels upstream to the reservoir in a time Tr < t < Tr , and at the same time, v adopts the value v = 0.

The reservoir is reached in a time t = Tr , and the pressure resumes the reservoir’s pressure head.

In a period of time Tr < t